linux-kernel - Re: [RESEND Patch] kcore: remove its pointless size

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Message-Id: <20090702094138.f86ead92.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Thu, 2 Jul 2009 09:41:38 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	ebiederm@...ssion.com (Eric W. Biederman),
	Amerigo Wang <xiyou.wangcong@...il.com>, tao.ma@...cle.com,
	linux-kernel@...r.kernel.org, adobriyan@...il.com,
	mtk.manpages@...il.com, Yasunori Goto <y-goto@...fujitsu.com>
Subject: Re: [RESEND Patch] kcore: remove its pointless size

On Wed, 1 Jul 2009 17:12:49 -0700
Andrew Morton <akpm@...ux-foundation.org> wrote:

> On Wed, 01 Jul 2009 16:25:05 -0700 ebiederm@...ssion.com (Eric W. Biederman) wrote:
> > Which is better than showing a random number of dubious relationship
> > to the size we normally show.  That code is just a maintenance problem.
> 
> Well it's not just that st_size is wrong before the first read.  It's
> also wrong after memory hot-add, up until the next read.
> 
And I found kclist_add() is not called at memory hotplug...


> > > If so, should we run get_kcore_size() in proc_kcore_init(), perhaps?
> > >
> > > In fact, do we need to run get_kcore_size() more than once per boot? 
> > >
> > > AFAICT we only run kclist_add() during bootup, so if proc_kcore_init()
> > > is called at the appropriate time, we can permanently cache its result?
> > >
> > > In which case get_kcore_size() and kclist_add() can be marked __init.
> > >
> > > Maybe that's all wrong - I didn't look terribly closely.
> > 
> > Memory hot add I expect is the excuse.  There is more that could be
> > done.  But this patch is an obvious bit of chipping away nonsense
> > code.
> 
> We have the infrastructure to get this right, I think:
> 
> - run
> 
> 	proc_root_kcore->size = get_kcore_size(...)
> 
>   within proc_kcore_init()
> 
yes, seems sane.


> - register a memory-hotplug notifier and each time memory goes online
>   or offline, rerun
> 
> 	proc_root_kcore->size = get_kcore_size(...)
> 
yes. and we need kclist_add() under memory hotplug.


> - stop running get_kcore_size() within read_kcore().
> 
> I suspect that read_kcore() will not behave well if a memory hotplug
> operation happens concurrently.  But that's a separate problem.
> 
> (hopefully cc's some memory-hotplug people)
> 
Maybe no problem. I don't think people does memory hotplug while he reads
/proc/kcore. (It sounds like modify coredump while investigating it.)

Thanks,
-Kame


> 
> Or we just leave /proc/kcore's st_size at zero.  It's a pretty hopeless
> exercise trying to get this "right", as nobody can safely _use_ that
> size - it can be wrong as soon as the caller has read from it.
> 
> 

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